The present invention relates to a permanent magnet alloy of the intermetallic compound type mainly composed of Nd and Fe, and more particularly to a Nd-Fe-B permanent magnet alloy having improved thermal stability.
Nd-Fe-B permanent magnet materials have been recently developed as new materials with higher magnetic properties than those of Sm-Co permanent magnets.
Japanese Patent Laid-Open Nos. 59-46008, 59-64733 and 59-89401, and Journal of Applied Physics, Vol. 55, No. 6, pp. 2083-2087 (1984) disclose that a magnet alloy having a composition of Nd.sub.15 Fe.sub.75 B.sub.10 corresponding to Nd(Fe.sub.0.88 B.sub.0.12).sub.5.7, for instance, has magnetic properties such as (BH).sub.max of about 35MGOe and iHc of about 10KOe, that the substitution of part of Fe with Co increases the Curie temperature of the magnet, and that the addition of Ti, Ni, Bi, V, Nb, Ta, Cr, Mo, W, Mn, Al, Sb, Ge, Sn, Zr or Hf leads to the increase in intrinsic coercivity iHc. The above maximum energy product (BH).sub.max (35MGOe) of such Nd-Fe-B alloys is much higher than those of rare earth-cobalt (R-Co) magnets which can be at most about 30MGOe.
These Nd-Fe-B permanent magnet alloys may be prepared by a powder metallurgy method. Specifically, raw materials for the magnets are melted in vacuum to form an ingot which is then crushed and pulverized, formed into a desired magnet shape in a magnetic field, sintered, heat-treated and then worked.
The sintering is performed in an inert gas such as Ar and He, in hydrogen or in vacuum at temperatures of 1050.degree.-1150.degree. C. The heat treatment conditions may vary depending on the types of rare earth elements used and the compositions of the magnets, but annealing is performed usually at about 600.degree. C. According to Sagawa, for instance, the annealing at 590.degree.-650.degree. C. provides high intrinsic coercivity iHc (nearly 12KOe). See J. Appl. Phys. 55(6), pp. 2083-2087 (1984).
However, Nd-Fe-B permanent magnet materials have extremely poorer thermal stability than conventional Sm-Co permanent magnets. For instance, when a magnet of Nd(Fe.sub.0.92 B.sub.0.08).sub.5.4 is heated to 140.degree. C., its intrinsic coercivity iHc irreversibly decreases by as much as about 65%. Thus, they have suffered from the problems that they cannot be assembled in automobiles and home electric appliances, and that they cannot be used in environments higher than room temperature.